Surveying instruments, surveillance cameras, and semiconductor manufacturing and testing devices require high-precision in their positioning control. Wide spread use of battery driven products have also lead to growing demand in both efficient operation and high-precision positioning. A positioning control unit is generally provided with a position feed back loop that generates a command rotational speed of the motor based on position error and a speed feed back loop that generates command current or command voltage of the motor based on speed deviation.
JP 2005-292898 A discloses a positioning control unit that generates command values to be delivered to a servo motor by combining a target position with corrected data generated in advance for absolute positioning control. The corrected data is generated by positioning the servo motor at predetermined constant angles, measuring the absolute positions of the specified positioning by a high-resolution encoder, and obtaining angle error data from position data of the high-resolution encoder and position data of the encoder of the servo motor.
The positioning control controls the motor by a processor driven digital control. In such case, data resolution (data length) and signal wave resolution of a PWM control circuit is constrained by hardware limitation of the processor, thus resolution of output torque pertaining to the motor is also constrained. Thus, regardless of remarkable improvement in resolution of position detectors of encoders, it has not been possible to achieve positioning of angle precision on the order of seconds (″), for example, by the conventional motor control unit.